1. Field of the Invention
This invention relates to control rods for a nuclear reactor and, more particularly, to an extension shaft interconnecting the control rod assembly to the control rod drive and means releasably securing the shaft from rotation about its longitudinal axis.
2. Description of the Prior Art
Typically, a nuclear reactor for the generation of electrical power comprises a longitudinally disposed cylindrical pressure vessel closed at both ends having a core of fissionable material which heats a primary coolant. The fissionable material is enclosed in elongated fuel rods assembled in bundles commonly called fuel assemblies. Moreover, a control rod assembly, i.e. a plurality of interconnected control rods, disposed in passageways or guide tubes within each fuel assembly is provided to control the reaction rate of the fissionable material. Each control rod assembly is moveably mounted within the fuel assembly passageways for motion into and out of the core for adjustment of neutron absorption in relation to the effective position of the control rod within the core. Thus, as the conrol rods are moving into the core, neutron poison material such as cadmium, boron, cobalt, or their alloys is introduced and the power level of the reactor is reduced. Moreover, in most reactors, complete insertion of the control rods within the core terminates the chain reaction of the fission process. Conversely, withdrawal of the control rods from the core reduces the poison level or neutron capture cross-section which increases the chain reaction. Therefore, accurate power regulation as well as complete termination of the chain reaction is accomplished by the control rod assembly.
For the commonly known land based pressurized water nuclear reactor, the heated primary coolant flows out of the nuclear pressure vessel and into at least one secondary coolant heat exchanger wherein the secondary coolant is heated to superheated steam conditions. In nuclear marine propulsion plants, however, the constraints of size, volume and mass of the nuclear system are, in the least, limited. For example, one type of nuclear system proposed for marine propulsion provides a pressure vessel that encloses the reactor core and, also, the secondary coolant heat exchangers. Accordingly, this consolidated nuclear steam generator or CNSG comprises; an elongated pressure vessel, a reactor core centrally disposed within the lower portion of the pressure vessel forming an annulus between the vessel and the core for primary coolant flow, an annular secondary heat exchanger vertically disposed above the core forming an elongated cylindrical plenum directly above the core and associated vessel equipment.
The operation of a nuclear reactor moreover periodically requires the removal of fuel rods from the core for refueling, maintenance, or the like. Accordingly, this requires the removal of the reactor vessel head together with the control rod drive systems mounted thereon. After the vessel head has been removed the reactor internals above the core must also be removed to expose the fuel assemblies which can then be retracted from their positions within the reactor core. Naturally, the reactor must be in a shutdown condition during this operation and maintenance of the shutdown condition is performed by each of the control rod assemblies completely inserted within the fuel assembly passageways or guide tubes provided therefor. Accordingly, means must be provided to disengage the completely inserted control rods from their respective drive systems during removal of the reactor head in order to prevent accidental withdrawal of a control rod from its fuel assembly during head removal. In the common pressurized water type nuclear reactor only one releaseable connection or coupling between each of the control rod assemblies and their respective drive systems is needed or provided. However, for the CNSG type nuclear reactor described above in which the reactor core is displaced substantially below the reactor vessel head with respect to a like sized core of a land based nuclear reactor, the problem of disengagement of the control rod assembly from its drive system is more complicated. The increased space above the core of this type CNSG reactor requires that the control rods be connected to their respective control drives by means of exceptionally long drive shafts. Moreoever, the refueling, maintenance or the like, disengagement or re-attachment of the control rod assemblies and their drive systems of this type (CNSG) reactor to the long drive shafts connected therebetween presents increased alignment and radioactive exposure problems, as well as additional space and storage problems. For example, with as many as 60 control rod drives each coupled to at least one control rod, but usually to a control rod assembly comprising a plurality of connected rods, through a "spider" arrangement, the storage, maneuverability and handling of the reactor head and associated long drive shafts is in the least burdensome. In addition, blind alignment, during head re-attachment, of the exceptionally long shafts to control rod assemblies displaced significantly distant from the reactor head or top portion of the nuclear vessel is exceedingly more difficult. As a result, it has been suggested to interconnect the control rods to an extension shaft coupled to the drive system. In this manner coupling and uncoupling of the reactor head and associated drive system would not vary from present methods and no exceptionally long shafts with their above mentioned problems would be attached thereto. However, an interconnecting extension shaft must also couple and uncouple with its respective control rod or control rod assembly. Moreover, it is necessary that coupling or uncoupling of the control rod drive to or from its extension shaft does not uncouple the control rod assembly from its extension shaft. That is, during the start up or shutdown procedure it is essential that accidental uncoupling of the control rod from its extension shaft is prevented.
Accordingly, it has also become necessary to provide interconnection means to the control rod assemblies which insure the complete insertion of the rods within the fuel assembly guide tubes provided therefor and which relieve the problems of alignment, storage, radiation exposure and the like as mentioned above. Moreover, the interconnection means must also provide securing means which prevent accidental uncoupling of the control rod assembly from its respective drive.